In electronic photography, the photosensitive member comprising a photoconductive element is in dark ambience in which an even electrical charge is given on its surface, wherein an electrostatic image is formed by means of an image exposure, and an electrostatic image is developed to produce a visible image.
Generally, the development method for an electrostatic image is roughly classified into the wet development process and the dry development process. The wet development process utilizes in the developing process a liquid developer in which ultra-fine particles of various pigments and colors are dispersed within an electrically insulating organic liquid. The dry development process utilizes the electroscopic fine particles known as toner which are composed of a coloring agent such as carbon black dispersed within a natural or synthetic resin. The dry development process is further categorized into several methods; the so-called fur brush method utilizing a developer composed exclusively of the above toner, the impression method and the powder cloud method. In addition, the so-called magnetic brush method and the cascade method are also available, which employ the developer in the form of a mixture of the carrier made of iron powder or glass beads and the above mentioned toner.
By carrying out such development methodes, the electroscopic particles, for example the electrically charged toner contained within a developer, deposits on the electrostatic image to produce the visible image. Such a visible image, without any change, is fixed on the phtosensitive member by heat, pressure or vapor solvent or, fixed after being transferred onto the paper or another base material.
The present invention relates to the carrier within a developer employed in the above mentioned magnetic brush method as well as the cascade method, and more specifically, relates to the carrier which gives required charge on the toner when it is agitated after being mixed with the toner.
The carrier is generally categorized into the electrically conductive carrier and the electrically insulating carrier.
As the conductive carrier an oxidized or non-oxidized iron powder is generally utilized. However, with the developer composed of an iron powder carrier, the triboelectric charge is not stably provided on the toner, as a further demerit, the visible image formed by the developer may be fogged. That is, in the course of employment of the developer the toner particles will deposit on the surface of the iron powder carrier particles causing a decrease in bias current due to increased electrical resistance on the carrier particles, resulting in a poorer definition and worse fogging of the visible image to be produced. For this reason, when the developer containing the iron powder carrier is employed in an electronic copying machine and if a copying operation is continuously carried out, the developer will deteriorate within a limited number of operational sequences and must be replaced or replenished so as to continuously obtain well-defined images.
With the typical insulating carrier, the surface of the carrier core composed of the ferromagnetic material such as iron, nickel or ferrite is evenly covered with insulating resin. When such an insulating carrier is incorporated into a developer, adhesion of toner particles on the surface of carrier is, unlike the case of the conductive carrier, minimized and at the same time the triboelectric charging between the toner and the carrier may be controlled, giving the developer rather longer service life. It is another advantage that such a developer may be employed in high-speed electronic copying machines. On the other hand with the insulating carrier, the coating covering the surface of the carrier core should be sufficiently friction resistant (durability), the coating should have sufficient adhesion-preventing characteristics so as to prevent the toner from forming a layer on the surface of the toner, in addition, the electrostatic charge with required intensity and polarity due to the friction between the carrier and the specific toner should be produced (electrostatic charge properties).
More precisely, when the insulating carrier within a developing unit is subjected to friction between other carrier particles, toner particles and inner surface of the unit, and the coating is worn due to the fricative movement, the stable electrostatic characteristic which is the product of the friction between the carrier and the toner will be gradually lost and consequently the required electrostatic charge cannot be given to the toner particles. Additionally, even if the coating of the insulating carrier the sufficient wear resistance, and if the adhesion of the coating to the core is improper, the electrostatic charge characteristic will be seriously damaged because the coating will peel off or collapse due to the fricative force mentioned above. Besides this, if the toner sticks to the surface coating to form a film, the result is again an unstable electrostatic characteristic. In such cases, and regardless of reasons, the entire developer should be prematurely replaced with a new developer.
In an effort to solve such drawbacks, the method to cover the surface of the core of the carrier with blended polymer (see Japanese Patent Publication Open to Public Inspection No. 110839/1979; hereinafter referred to as Japanese Patent O.P.I. Publication) is known as well as the carrier in which the surface of the core material is covered with copolymer of vinylidene fluoride/tetrafluoroethylene or covered with such a copolymer along with a second blended polymer. (See Japanese Patent O.P.I. Publication No. 208754/1983, No. 176048/1985, No. 176049/1985, No. 176050/1985, No. 176051/1685, No. 176052/1985, No. 176053/1985, No. 176054/1985, No. 176055/1985.)
Also the carrier using the acrylate having fluorine atoms in a side chain as coating resin was disclosed (see Japanese Patent Examined Publication No. 16617/1985). The applicant, prior to these publications, also proposed a similar art in regard to the like carrier in the specifications attached to Japanese Patent Application No. 240758/1984, No. 240759/1984, No. 240763/1984 and No. 240762/1984.
However, the carrier coated with copolymer of vinylidene fluoride/tetrafluoroethylene has a certain demerit, as the soft resin is responsible for a shorter service life due to the wear and tear of the coating layer.
Additionally, in the carrier coated with a blended polymer composed of the copolymer of vinylidene fluoride/tetrafluoroethylene along with the second polymer, the blended second polymer remarkably improved the bonding between the coating and the core, besides which, the much harder resin itself contributed greatly to a longer service life. However, the blending of the second polymer constituted a drawback; a smaller or unstable electrostatic charge.
With the carrier coated with an acrylate which has fluorine atoms in side chain, the electrostatic charge is defectively insufficient. With the carrier disclosed in Japanese Patent Examined Publication No. 16617/1985, for example, the electrostatic charge was apparently insufficient. Even with the carrier disclosed in the Japanese Patent Application No. 240758/1984 submitted by the said applicant, the electrostatic charge was still insufficient though it was much greater than that of the above mentioned carrier and the electrostatic charge became unsatisfactorily unstable in the course of an extended copying operation.